Log vtcombo output to stderr
Log vtcombo output to stderr

This makes it so much easier to debug issues rather than logging into
the docker image and trawling through some log files. Our test
automation also automatically deletes the docker image so it's hard to
get to the log file after the fact.

Signed-off-by: Jon Tirsen <a563f38b2a987e2aee263634a499e77cd89fe2fc@squareup.com>

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